1. Field of the Invention
The present invention relates to an information recording medium such as a CD and a DVD.
2. Description of Related Art
There is known a disc such as a CD-R (Compact Disc-Recordable), a DVD-R (a DVD-Recordable) and a DVD-RW (a DVD-Re-recordable) as an information recording medium on which information is recordable and rewritable.
A groove track (a track for recording) slightly wobbled in a disc radial direction is preformed on those discs in order to record information in an unrecorded area. The groove track is formed on the disc, according to a wobble signal produced by frequency-modulating a carrier having a predetermined frequency by using pre-information indicating position information of the track on the disc.
In order to extract the wobble signal from the groove track, a reflected light from the groove track of an irradiated optical beam is received by a photodetector divided into two detecting elements with a parting line optically parallel with a tangential direction of the groove track, and a differential signal of the outputs from those two detecting elements is produced and supplied to a BPF (Band Pass Filter) having the above-mentioned predetermined frequency as a center frequency.
The disc is rotation-controlled so that an average frequency of the wobble signal extracted via the BPF is equal to the above-mentioned predetermined frequency. The wobble signal thus extracted (hereafter, it is called “extracted wobble signal”) is utilized as a reference signal for generating a recording clock signal. Namely, the extracted wobble signal is a continuous signal having a frequency component which is synchronized with the rotation of the disc, and the recording clock signal which is accurately synchronized with the rotation of the disc is generated by generating the clock signal synchronized, in phase, with the continuous signal.
In addition to the wobbled groove track, so-called prepits are formed on a land track neighboring the groove track at a predetermined interval. Since the prepit is formed on the land track, it is also called “Land Pre-Pit” and “LPP”, and the prepit is sometimes called “LPP” in this specification. The prepit is also utilized for phase-adjustment of the recording clock signal, which is generated from the extracted wobble signal. Since the prepit is preformed at the predetermined interval in an information recording area on the disc, an information recording and reproducing apparatus generates the recording clock signal based on the extracted wobble signal, and at the same time, the apparatus adjusts the phase of the recording clock signal based on a detecting result of the prepit. In that way, the information recording and reproducing apparatus generates the accurate recording clock signal in order to record information on the disc and performs information recording based on the signal.
In some discs, the groove tracks for recording information are formed discontinuously (intermittently) in the lead-in area in which the information including the information about a copyright and the information for preventing an illegal copy is recorded. This area is sometimes called “embossed area”. Namely, though the groove track is continuously formed in a data area for recording information, the groove track is discontinuously formed in the above area within the lead-in area. As a result, the configuration of the disc surface is such that convex and concave portions are repeatedly formed.
As to an unrecorded disc such as a DVD-RW, the important information such as information for preventing the illegal copy is recorded in advance in an embossed area in the lead-in area, and needless to say, the information is readable. However, since the groove tracks for recording are intermittently formed in the embossed area, even when the predetermined information is recorded in order to illegally re-write such information, the re-written information cannot be reproduced correctly. In that way, the important information as to preventing the illegal copy and the like is prevented from being re-written.
Next, a recording format of the DVD-RW will be concretely described. FIG. 1A schematically shows a cross section of the DVD-RW, and a lead-in area 51, a data area 52 and a lead-out area 53 are provided from inside to outside of the disc. The lead-in area 51 is the area in which various control information and management information are recorded, and the data area 52 is the area in which video information and the other recording information are recorded. The lead-out area 53 is the area which prescribes the outermost portion of the data area 52.
FIG. 1B shows a more detailed data configuration of the lead-in area 51. The lead-in area 51 includes, from inside of the disc to outside, an initial zone 511, a buffer zone 512, an RW physical format information zone 513, a reference code zone 514, a buffer zone 515, a control data zone 516 and an extra border zone 519.
The control data zone 516 includes a control data block 517 located inner side and a servo block 518 located outer side thereof. Inner side of the control data zone 516 is an embossed area in which the embossed pits are intermittently (discontinuously) formed in a circumference direction (the tangential direction) of the disc on the groove track for recording the information.
It is noted that, in most discs, embossed pits readable as an RF signal are formed in the control data block 517, and embossed pits unreadable as the RF signal are formed in the servo block 518. Thus, the control data block 517 and the servo block 518 are respectively expressed as a readable embossed area 517 and an unreadable embossed area 518 for convenience afterward.
In the readable embossed area 517, control data such as information about the copyright and information for preventing the illegal copy is pre-recorded before the shipment of the disc from a factory. The control data is recorded as the deep embossed pits which can be read out by the recording and reproducing apparatus of the optical disc. However, since the embossed pits are intermittently formed in the readable embossed area 517, the control data cannot be read out correctly even though the data is rewritten. As a result, illegal rewriting of the control data can be prevented. It is noted that the prepits are not formed on a land track L in the readable embossed area 517 in most DVD-RWs.
On the other hand, unreadable embossed pits are formed in the unreadable embossed area 518 in order to prevent reading out the information which is rewritten and recorded in the area. In most DVD-RWs, the embossed pits formed in the unreadable embossed area 518 are shallower than the embossed pits formed in the readable embossed area 517. Also, the prepits are formed on the land track L in the unreadable embossed area 518 as well as on the land track in the data area. It is noted that the prepits in the unreadable embossed area 518 are formed in order to establish clock synchronization for writing the information in the extra border zone 519 following the unreadable embossed area 518. When the information is written on the DVD-RW, the information is recorded based on the recording clock signal which is generated from the extracted wobble signal and the detection signal of the prepit. However, since the prepits are formed discontinuously, the recording clock signal having an accurate phase cannot be generated without reading the area of the DVD-RW for a certain length to read out plural prepits.
On the other hand, in order to record the information in the extra border zone 519, the accurate recording clock signal has to be generated at the head of the extra border zone 519. So, the plural prepits are detected in the unreadable embossed area 518, which is formed just in front of the extra border zone 519, and the accurate recording clock signal is generated. In that way, when a light beam for recording arrives at the head of the extra border zone 519, the accurate recording clock signal has already been generated.
In this view, in the unreadable embossed area 518, no information is recorded, and the prepits are merely formed according to the predetermined rule. Thus, the recording and reproducing apparatus generates the recording clock signal of the accurate phase, which is described below, by reading out the prepits in the unreadable embossed area 518 to record the information in the extra border zone 519 based on the recording clock signal.
As described above, no prepit is formed in the readable embossed area in the control data zone because the embossed pits and the prepits interfere with each other in the embossed area, and in many cases, neither the embossed pits nor the prepits can be read out correctly. This is why the unreadable embossed area which follows the readable embossed area is formed on purpose to read out the prepits.
However, as to the unreadable embossed area, conditions for manufacturing are strict. Concretely, some conditions have to be satisfied at the same time, e.g., (1) making it possible that the prepit signal is accurately read out by keeping the level of the prepit signal, (2) making it possible that a push-pull signal is accurately output in order to execute tracking control by a recording apparatus, (3) making it possible that a DPD (Differential Phase Detection) signal, which is one of tracking servo systems in the DVD-ROM, is accurately output in order to execute the tracking control by a reproducing apparatus. So, in the unreadable embossed area, the pit deeper than the groove and shallower than the embossed pit is formed on the disc so that each level of the prepit signal, the push-pull signal and the DPD signal satisfies a standardized value. Thus, very precise adjustment is needed in cutting.
In addition, if such a new format that the ROM area is formed for some purposes inside the disc of the DVD-RW, in addition to the control data zone is proposed, the unreadable embossed area has to be formed after the readable embossed area, i.e., the ROM area, without exception, like the case of the above-mentioned control data zone. The unreadable embossed area, as described above, is formed in order to generate the recording clock for the following recording area, and hence no information can be recorded therein. Namely, recording capacity of the disc is decreased by the quantity of the unreadable embossed area thus formed.